1. Field of the Invention
This invention relates to method and apparatus for alignment of members, and more particularly to the method and apparatus for aligning a photomask (hereinafter called "mask") and a wafer in semiconductor circuit pattern exposure.
2. Description of the Prior Art
Conventionally, it is known to scan alignment marks provided on a mask and wafer, respectively, with a laser beam condensed as a spot, when the mask and wafer are to be aligned with each other in the semiconductor circuit pattern exposure operation, as disclosed in Japanese Laid-Open Patent Application No. 90872/1983 and others. FIG. 1 shows one of alignment marks provided on the mask or wafer. In this Figure, designated by reference characters M1, M2, M1' and M2' are the mark elements forming the alignment mark on the mask, and among those the mark elements M1 and M2 are inclined at the same angle with respect to the beam scanning line L, while the mark elements M1' and M2' are inclined oppositely but at the same angle. The angle between the mark elements M1 and M2', and the angle between the mark elements M2 and M1' are the same, for example, 90 degrees.
On the other hand, designated by reference character W1 and W1' are mark elements forming the alignment mark on the wafer. The angle between the mark elements W1 and the mark elements W1' is the same as the angle between the mark elements M1 and M2'. Therefore, when the mask and wafer are aligned, the mark element W1 extends parallel to the mark elements M1 and M2, while the mark element W1' is parallel to the M1' and M2', and in addition, the intervals therebetween are the same. The scanning beam is diffracted by the edges of the mark elements, but the optical system includes means for spatial frequency filtering on the Fourier transform plane with respect to the mask and wafer to block the light other than the signal light. By this function, a photoelectric detector only receives the diffracted light from the alignment marks, and therefore, produces a series of good signals.
A signal processing system processes the signal to determine the displacement between the mask and wafer in X, Y and .theta. directions. In accordance with the determination, the wafer or the mask is moved to correctly align them. However, when the marks are scanned by a spot-like beam, the detection may be significantly affected by dust possibly existing in the optical system, a score in the optical elements in the system or the like. Additionally, when there is a piece of dust or something like that on the scanning line, as shown by reference numeral 8 in FIG. 1, or when a part of the alignment mark is missing, the alignment is achieved only with lower precision or cannot be achieved at all.
Further, when the surface to be scanned is rough, such as a rough aluminum surface, the beam is
reflected and scattered by the portions other than the mark portions, and this scattered light is not filtered out and reaches the photoelectric detector as noise so that the alignment operation is sometimes disabled. To obviate the problems, the Applicant of the present application has proposed, in Japanese Patent Application No. 181315/1981, that the alignment marks are scanned by a sheet-like beam (a beam which forms a thin and elongated illumination area on the surface to be scanned). The broad area, provided by the sheet-like beam, with which the marks are scanned, is effective to significantly reduce the influence of dust 8 on the scanning line or the defects of the alignment marks 9.
FIGS. 2 and 3 show the configuration of the filter for effecting the spatial frequency filtering on the Fourier transform plane with respect to the surface to be scanned. FIGS. 2 and 3 show the configurations for the spot-like beam and sheet-like beam, respectively. When the illumination area of the beam on the surface to be scanned extends in the direction perpendicular to the direction of scan, the diffracted light which has been diffracted by the edge portion of the alignment mark, i.e., the signal light, forms a spectrum distribution concentrated in a narrow area at the Fourier transform plane. And, it is possible to narrow the transparent part 7 of the filter as shown in FIG. 3. On the other hand, the randomly scattered light which is the noise component is incident over the entire surface of the filter. Thus, the influence of the ghost and the flare caused by the structure of the optical system, or the noise component resulting from the randomly scattered light from the wafer having a rough surface can be avoided without decreasing the amount of signal light so that a signal detection with good signal-to-noise ratio can be provided. It will be understood that the use of the sheet-like beam provides advantages more than those provided by the aggregation effect by enlargement of the beam.
Furthermore, Japanese Patent Application No. 208764/1982 proposes that sheet-like beams crossed at their inclination to meet the angle between the mark elements be used for the scan of the alignment marks. However, in one of the embodiments disclosed in the Application, the sheet-like beams are made to have different polarizations for the discrimination of the alignment marks having different inclination, which results in less latitude of design.